Slack's Agents & AI Apps feature ships a native streaming surface that
renders a live-typing message instead of the edit-based progressive
updates the adapter used until now.
The adapter now implements the existing draft-streaming interface:
- supports_draft_streaming() opts in whenever the app is connected and
native streaming hasn't been detected as unavailable.
- send_draft() starts a stream on the first frame (chat.startStream,
anchored to the resolved thread_ts, with recipient_team_id/user_id
for channel streams) and appends only the delta on subsequent frames
(chat.appendStream is append-only). The consumer's trailing cursor
glyph is stripped before delta computation.
- Unlike Telegram drafts (ephemeral, replaced by a real sendMessage),
a Slack stream IS the final message. send() therefore intercepts the
turn-final delivery for a chat with an active stream whose streamed
text is a prefix of the final content, and seals it via
chat.stopStream with the remaining delta instead of posting a
duplicate. Rich Block Kit (when enabled) is applied to the sealed
message via chat_update, mirroring the finalize path in edit_message.
- Feature-gate errors from chat.startStream (not_allowed,
missing_scope, unknown_method, ...) are cached on the adapter so
subsequent runs skip straight to edit-based streaming with a single
warning naming the fix (enable Agents & AI Apps for the app);
transient errors only disable drafts for the current run via the
consumer's existing send_draft failure handling.
- Segment breaks (new draft_id) and disconnect() seal any open stream
so chats are never left with a dangling live-typing indicator.
No consumer or config changes: streaming.transport auto/draft now
lights up native streaming on Slack through the same interface
Telegram drafts use, and the edit-based path remains the fallback.
Will Lynas